In a typical automatic transmission system for automotive vehicles, a mechanical motion transmitting linkage or cable mechanism is provided between a driver-actuated range selector lever and a control valve assembly for controlling ratio ranges in the transmission. It is possible to eliminate the mechanical linkage or cable mechanism between the range selector lever and the transmission by providing a driver-controlled switching system that distributes range selector signals to an electrohydraulic control valve assembly for the automatic transmission. An example of such a driver-controlled electronic switching system may be seen by referring to U.S. Pat. No. 5,696,679, which describes an integrated electronic control for moving a transmission park pawl into and out of engagement with a park gear carried by the driven shaft of an automatic transmission. U.S. Pat. No. 5,696,679 is owned by the assignee of the present invention.
The elimination of a mechanical linkage or cable mechanism that operates a range selector valve for an automatic transmission makes it necessary to introduce into the system multiple control redundancies to account for the possibility of system faults. This ensures that forward drive will be achieved when the driver commands forward drive, and that reverse drive will be achieved when the driver commands reverse drive. The same is true when the driver selects a neutral mode.
If an error occurs in any of the functions that are selected by the driver, there is a need for the driver to be alerted to the presence of the error so that appropriate adjustments can be made in the control system or in its calibration to eliminate the error. This alert is needed even though the presence of the error may not result in an erroneous drive range selection following a command by the driver for a particular drive range (i.e., forward drive range or reverse drive or neutral). There is a need also for the transmission system to default to the neutral state when multiple errors are indicated and error signals are fed back to the driver.
The use of a mechanical linkage between a driver-actuated range selector lever and a range control valve in conventional transmission controllers presents space and packaging problems for components located in the underbody area of the vehicle. Further, such mechanical linkages are subject to wear and possible malfunctions. They also may be the source of noise, vibration and harshness problems (NVH), especially after the mechanical linkage elements exhibit wear.
Remote range selector systems for automatic transmissions in automotive vehicles usually are not integrated within the transmission itself. Prior art systems that do not include a mechanical linkage mechanism (e.g., shift-by-wire transmission control systems) may be so-called "add-on" designs that require space within the underbody area of the vehicle. This compromises the ability of the vehicle designer to use efficiently the space in the underbody area for other purposes. In the case of transaxle transmissions for front wheel drive vehicles, the limited space within the engine compartment may present the designer with packaging problems. Furthermore, conversion of a left-hand drive vehicle to a right-hand drive vehicle, and vice versa, becomes more difficult.